The present invention is directed to a method for non-destructively inspecting the wall thicknesses or strength of a component, where the dimensions of the component or of the component surface are measured, substantially without making any contact, and are described by digital data.
Mechanically and thermally stressed components, such as aggregate parts (cylinder heads, shafts, etc.) or engine components (e.g., turbine blades), need to be inspected to check for adherence to minimum wall thicknesses. The inspection procedures for such components often specify a minimum value that the wall thickness must not fall below anywhere on the component. At the present time, the wall thicknesses of cylinder heads, for example, are examined by sawing the component into individual parts in order to make all locations accessible and by subsequently performing a manual inspection using a special dial gauge. Besides the fact that the component is destroyed, the main drawback of this method is that the lack of a reference to the component""s coordinate system makes it difficult, in the event of a defect (thin wall), to infer the cause of the defect (e.g., displacement of a sand core if the component is a cast part).
The firms BIR, SMS and Aracor, etc. have computer tomography systems for the non-destructive inspection of components. Computer tomography provides a stack of two-dimensional gray-value sectional views through the component which can be individually displayed on a computer. In addition to these systems, there is also system software for examining selected wall thicknesses in individual, two-dimensional gray-value sectional views. The inspection is performed interactively with the user, i.e., a complete inspection is not automatically possible. The main disadvantage of this method is that the actual wall thicknesses may be smaller than the two-dimensional sectional views, since the section is generally not perpendicular to the wall.
An object underlying the present invention is to devise a method for non-destructively inspecting the wall thickness or strength of components which will make it possible to automatically capture, in reliable fashion, all actually occurring wall thicknesses, and which will enable the user to quickly evaluate the components and/or critical wall thicknesses.
The present invention provides a method for non-destructively inspecting the wall thickness or strength of a component, where the dimensions of the component or of the component surface are measured, substantially without making any contact, and are described by digital data, wherein a computer implements the following: (a) a multiplicity of the component""s surface points, which substantially completely describe the component, is automatically calculated in a three-dimensional coordinate system; (b) starting from each of the surface points, going out in one direction that runs substantially perpendicularly to the surface of the component at the surface point, into the material, one scans for at least one opposite surface point; (c) the wall thickness of the component at the surface point is ascertained as the smallest distance between the surface point and the at least one opposite surface point; and (d) the component is visually displayed and, in the visual display, surface points are highlighted for which the wall thickness falls below and/or exceeds one or more predefined values.
Thus, starting from each of the surface points, going substantially in a normal direction into the material, one scans for surface points on an opposite surface, to reliably find the smallest material thickness everywhere.
Here, xe2x80x9csubstantially in a normal directionxe2x80x9d signifies that, proceeding in a normal or perpendicular direction into the material, a target point on the opposite surface is initially sought and acquired. Then, within a tolerance range to be preset around the target point, other points are selected, and their distance to the starting point is defined. The smallest distance indicates the particular material thickness.
At first glance, this type of search may not provide the actual material thickness. For example, when the component is bounded on mutually opposing sides by surfaces having different curvatures, it may occur that the search from one side does not provide the smallest material thickness. In such a case, however, one obtains the correct material thickness from any surface point on the other side, since the calculation is made for all surface points. Thus, the actual material thickness to one surface point is the smallest value that one obtains starting from this surface point or from any other surface points, in the direction of the first considered surface point.
The method of the present invention can be implemented fully automatically. The user merely needs to enter the limit values for the wall thickness, the component""s adherence to which is to be tested. In the visual representation, for example in a screen display, all surface points are then highlighted, for which the calculation revealed that the wall thickness falls below and/or exceeds the limit values. For example, in a three-dimensional black-white display of the component on the screen, those locations can be marked in color where the wall thickness is smaller than a preset minimum value or where the wall thickness is between an upper and a lower limit value. Particularly when the depicted component is transparent or is rotatable on the screen, the user can very easily recognize whether the component has any regions having a critical wall thickness.
The three-dimensional coordinate system used for visualizing the component is advantageously the same as the one containing the component""s measurement data subsequent to the measurement, for example Cartesian coordinates or cylinder coordinates. The uniform coordinate system makes it possible to draw from ascertained anomalies (e.g., thin walls) to specifically infer the causes of the defects (e.g., displacement of a sand core if the component is a cast part).
The component is precisely measured in three dimensions, preferably using optical 3-D coordinate metrology (e.g., laser scanners, strip-projection sensors, etc.) or tomographic measuring instruments (e.g., X-ray computer tomographs). By properly selecting the measuring instrument, one can thoroughly measure the dimensions of the components, including any existing internal structures. The result is a digital representation of the component, which can exist in one of the following forms: (a) a stack of three-dimensional, gray-value sectional views through the component or a three-dimensional voxel data record (a voxel is a small element of volume having a gray value, which is a measure for the density of the component in this element of volume); (b) a dense point cloud, which describes the surface of the component; (c) a triangulation, which describes the surface of the component.
Each of these three cases provides a digital description of the actual state of a component upon which to base the automatic analysis of the component""s wall thicknesses.
In the first case, from a voxel data record, one calculates surface points of the component, utilizing the fact that the gray value of the voxels at the surfaces of the component generally does not change abruptly from one value to another.
This means that, as surface points, one takes, for example, the midpoints of voxels, which have a gray value that lies within a predefined range between the gray value of the material of the component and the gray value of regions in which there is no material. These points, which lie on or in the vicinity of an ISO gray-value surface, form a dense point cloud.
In the case that the component is described by a stack of three-dimensional gray-value sectional views, one can perform the above calculation analogously on the pixels of the gray-value sectional views, and subsequently describe the locations of the obtained surface points in the three-dimensional coordinate system. Or one first combines the gray-scale sectional views and obtains a three-dimensional voxel data record in which each voxel corresponds to one small volume in the coordinate system used and has an associated gray value. For this, commercial software modules are already available, e.g., the software MIMICS of the firm MATERIALISE or the software VG STUDIO MAX of VOLUME GRAPHICS.
A local tangential plane is then determined for each surface point using a correction method, with the assistance of other surface points in its vicinity. The direction in which one subsequently scans for other component surfaces is the local normal on the local tangential plane, i.e., the normal vector of the tangential plane which points into the material.
In the second case, where the measuring technique calls for describing the surface of the component by a dense point cloud, a correction method is used for every surface point to determine a local tangential plane, whose local normal is given by the normal vector of the tangential plane.
In the third case, where the measuring technique calls for describing the surface of the component by a triangulation, any points of the triangles, but preferably the centroids of the triangles, may be taken as the surface points of the component, the local normal, i.e., the search direction for each surface point, being given, for example, by the normal vector of the corresponding triangle.